Fix incorrect code snippet description for `just-index`

[palashmon]

Updated the readme file for just-index to reflect that the code snippet throws an error instead of returning undefined, which was originally stated. This change ensures that the code snippet description is accurate and helpful for developers.

Code snippet:

index([], null);    // throws error
index({}, 'id');    // throws error

URL:

• https://anguscroll.com/just/just-index

[angus-c]

thank you!